Morphological Variation between Geographically Disjunct Populations of Atlantic Salmon: The Effects of Ontogeny and Habitat Shift

Abstract

Summary 1. Previous studies suggest that morphological divergence among populations is a widespread result of reproductive isolation and local adaptation. In this paper I test the hypothesis that morphological differentiation should be reduced when individuals shift from a geographically heterogeneous habitat (benthic, fresh water) to a relatively uniform habitat (pelagic, open ocean). 2. Juvenile Atlantic Salmon Salmo salar from two disjunct populations but reared under the same environmental conditions differed in a number of morphometric traits and general body shape in both the freshwater and the premigratory stage associated with the oceanic phase. 3. Relative to the premigratory phase, the riverine forms of these populations differed in a larger number of variables, the discriminant functions correctly assigned a larger proportion of individuals to their source population and morphological distances between population centroids were also greater. 4. Morphometric distances between the centroids of stages within populations were greater than distances between populations within stages. 5. Though preliminary, these results suggest that, for some traits, isolated populations may diverge in a determined stage but converge in another, especially in species that undergo a habitat shift.